Tire Pressure Monitoring Systems, abbreviated as TPMS, are mainly used to automatically monitor tire pressure and provide an early warning of low tire pressure caused by air leakage or high tire pressure caused by high temperature so as to avoid tire rupture and ensure safety driving.
Generally, a tire pressure monitoring system consists of a transmitting and detecting unit (one for each tire) and a receiving and display unit (only one):
The circuit of the transmitting and detecting unit consists of a temperature and pressure sensor, a MCU (Main Control Unit), a transmitter, and a battery. The unit includes a protective casing that protects the electric circuit and includes a device for fastening the casing on the run of the tire.
The circuit of the receiver consists of battery unit, a receiver, a MCU and a display unit. The structure for the circuit comprises a casing, a function switch set on the surface of the casing and a display screen.
As shown in FIG. 1, an automobile tire pressure monitoring system generally consists of a transmitting and detecting unit 92 and a receiving and display unit 91, which thereupon comprises display unit 9101, receiving unit 9102 and receiving antenna 9103.
The left side of FIG. 1 is a front sketch diagram of the display unit, i.e., the screen that is displayed on the automobile instrument panel. The display unit 9101 of an ordinary automobile includes four data display areas 91011, that display the parameters of the four tires respectively. The broken lines in FIG. 1 represent the correlations between the display area 91011 and the respective tires.
The operation process is as follows: The sensor in the transmitting and detecting unit 92 onverts variations of the tire pressure into electric parameters which vary accordingly to electronic component induction. Then the electric parameters are processed by a MCU in the transmitting and detecting unit into digital code signals. After identification of the ID of the digital code signals in this unit (used to distinguish it from other units) is completed, these code signals are transmitted via a carrier frequency by a transmitter. The original data is recovered after the radio signals are received and demodulated by the receiver antenna 9103. Then, after being processed by the MCU of receiving and display unit, the data is displayed on the corresponding tire data area of the user's interface by the display screen installed in the vehicle. In this way, the driver can clearly know the pressure in each tire. When the received data shows the pressure in the tire is lower or higher than the set limit, the MCU will show an alarm icon on the display screen. The driver can then take appropriate action for the tire according to the data of tire pressure shown so as to ensure safe driving.
One transmitting and detecting unit is available for each of the four tires. In the MCU memory of the transmitting and detecting unit, the ID code of that unit has been set (Used to distinguish it from other identification codes). The identification code of the unit is provided in the signals. Through comparing an ID code that is pre-saved in the MCU memory with the received ID code, MCU of the receiving and display unit identifies the relation between IDs and determines which tire the data is associated with. After processing, the corresponding pressure and temperature information on the respective area will be displayed on the screen. With the judgment for ID codes, the receiving and display unit can determine the received information sent by other transmitting and detecting units to be unwanted and discard it.
The tire transposition is usual:
Because of the different loads on the four tires, caused by the front-wheels being responsible for veering, and the rear and front axle hanging in different angles, the degrees and the positions of abrasion for each tire are usually different. In order to improve the stability while turning, the rear wheels of most front-driving and front-veering vehicles have a larger tilt angle than the front wheels taking an outward shape of Chinese character  (Several kinds of vehicles take an inward shape of Chinese character  which is decided by the hanging design.). As a result, the rear wheels are liable to abrade on the inside and outside rim, i.e., the section that forms a trapezoid. Similarly, the same abrasion may occur to the rear wheels. In order to prolong the service life of the tire, and for the purpose of four tires abrading synchronously and equably, regular tire rotation is required.
In the process of tire rotation the corresponding transmitting and detecting units will be transposed. This will cause the original correlation between ID code saved in MCU memory of the receiving and display unit and the tire identification information to be miscorrelated so that the information displayed on the screen about the pressure and temperature as correlated with the tire is wrong.
When a new tire is installed, or when the transmitting and detecting unit on a tire is replaced as it becomes damaged, the ID code of the new unit will be different from the original ID. Consequently, the identification information of the ID code saved in the MCU memory of the receiving and display unit and the corresponding tire is no longer applicable to the ID code of the newly replaced unit. The receiving and display unit will discard the information received from the replaced unit. The pressure and temperature information sent by the new unit will not be displayed on the screen.
Nowadays, four methods are mainly adopted in the world to solve the problem of tire identification during tire transposition and replacement. The technologies and their respective characteristics for the four identification methods are as follows:                1 Fixed encoding method: The correlation between ID code in the MCU memory of the receiving and display unit and the tire identification information is fixed at the factory. The same ID code is also fixed in the MCU memory of the transmitting and detecting unit and is marked on the surface of the transmitting and detecting unit. During the installation, the transmitting and detecting units are installed on the corresponding tires in accordance with the marks and no change is allowed during application. This method is quite simple and its shortcoming is that wrong installation is not allowed. Otherwise, identification confusion may arise. Meanwhile, if a transmission unit is damaged, the user has to go to the manufacturer for repair or replacement. The transmitting and detecting units must be reinstalled in accordance with their marked positions when the tires are rotated.        2. The method of interface input: This kind of identification technology is to print the ID code of each transmission unit on the packing of the product or on the product itself. However, after a tire is rotated or a transmission unit is damaged, the ID code should be input into the receiving port to make a correct identification. The shortcoming of this method that since the ID codes are 16 or 32 digits, the input procedure is complicated and the user is liable to mis-input the code set. Furthermore, the numerous buttons cause inconvenience and awkward layout in the automobile.        3. The method of low frequency (LF) wake-up: This technology makes use of the near-field effect of the LF signals (for example 125 KHZ). As shown in FIG. 2, there is a LF antenna 9104 near each tire in this scheme. By sending LF signals to the LF antenna near the corresponding tire, the monitoring system can trigger the corresponding tire's transmitting and detecting unit 92 respectively, which transmits the ID code through RF. The receiving unit 9102 obtains the corresponding identification code of the tire's TPMS unit via this signal and thus automatically locates the tire. The shortcomings of this method are that: 1) Four LF antennae 9104 should be installed near the corresponding tires. To install and lay out the wires involves a considerable amount of work; 2) The LF signals may mistakenly trigger an adjoining transmitting and detecting unit; and 3) Due to the complicated electromagnetic environment and disturbances, the LF signals can be disturbed, which leads to the ineffectiveness of the identification process.        4. The method of antenna near field receiving: As shown in FIG. 3, this technology includes four receiving antennae of the receiving and display unit 9103 which stretch over the near field within 20-30 cm around each tire and are controlled by numerical controlled microwave switch 9108. When the information sent by the transmitting and detecting unit of a certain tire needs to be received, it is only when the microwave switch of the receiving antenna near that tire is on and the others are off that the pressure and temperature of the tire, which the RTPM unit is connected to, can be displayed on the receiving screen. The shortcomings of this identification technology are that: 1) The layout of the antennae is complex and the microwave switch is costly. Moreover, the isolation of the RF switches is not good enough at the present time, so there exists the possibility of mixed code, i.e. receiving the information from other tires; and 2) Due to the complicated electromagnetic environment and all kinds of disturbance in an automobile, the LF signals will be disturbed, which will lead to the ineffectiveness of the identification process.        
To summarize, the above-mentioned identification technologies have one or more of the defects as follows:                1. The transmission unit installation cannot be displaced. So faulty transmission units have to be sent to the factory for burning the corresponding identification code.        2. It is a tedious job to input the identification code. The users may mis-input the identification code.        3. The design or the layout may be influenced.        4. Four LF antennae are required to be installed, which will cause difficulties.        5. LF signals may mistakenly trigger adjoining transmitting and detecting units.        6. The antenna design is complex, the RF switch is costly, and RF isolation is not sufficient.        